TTL, Time To Live.
8 bits.
A hop count that must be decremented every time a node forwards a packet.
If the TTL reaches zero it is stripped off the ring. This allows for a total node space
of 256 nodes on a ring. However, due to certain failure conditions (e.g. when
the ring is wrapped) the total number of nodes that are supported by SRP is 128.
When a packet is first sent onto the ring the TTL should be set to at least
twice the total number of nodes on the ring.

P, Parity. 1 bit.
This bit is used to indicate the parity value over the 15 bits of the SRP header
to provide additional data integrity over the header. Odd parity is used (i.e.
the number of ones including the parity bit shall be an odd number).